Exactly! It was driving me totally nuts to the point where I thought I had a defective right-side IEM. Sometimes it closes off completely, but other times it's just a rolloff in the treble that starts somewhere in the 6k-8k region. It makes things like cymbals, hi-hats, and other sparkly sounds shift to the left side of my head. Very disorienting.
Only solution I've found so far is to try and get a very shallow and loose fit. Not ideal, especially for things like walking which immediately shakes the IEM loose from its very tenuous seal. Sounds like I just need to keep searching for better fitting tips...
It's not quite that. If the seal is especially secure on the right, it's like it gradually keeps suctioning into my ear until the sound closes off entirely either from my ear canal closing or from the tip's bore hole closing (not sure which). But most of the time, even if I have only a moderate seal and moderate insertion depth, I get treble roll off (and only ever on the right side). I can manually push the face plate of the IEM forward (prying open my ear canal front to back) which gives me perfect clarity on that side, but obviously I can't just hold it there all the time. Dunno. It's a puzzle.
Happens with any one of the dozen or more tips I've collected now, including more unusual ones like double-flange tips. And it happens on both my longer nozzle IEM (though not as severely) and my short nozzle IEM.